With computer systems in organizations providing a service for financial institutions and the like which demand continuous operation 24 hours a day, 365 days a year, a technology known as remote copying is widely employed which duplicates logical volumes of data between two storage apparatuses located in sites remote from one another but coupled via WAN (Wide Area Network) to enable continuous operation.
With computer systems which perform remote copying, data is duplicated between a master volume in a storage apparatus at the main base and a duplicate volume in a subsidiary storage at a backup base. Thus should any damage occur to the main storage, the subsidiary storage can continue to provide a processing service, enabling an organization such as a financial institution to have a computer system that operates continuously.
With the expansion in the quantity of data and the increased performance of storage apparatuses in recent years, there has been a continuing increase in the quantities of data transferred via WAN during remote copying. Technology such as that disclosed in PTL 1 (Japanese Patent Application Publication No. 2002-358222), for example, can be cited as a technique for reducing the quantity of data transferred via WAN during remote copying.
With the technology disclosed in PTL 1, the main storage is provided with a copy volume which, at the point where data is synchronized between the master volume and the duplicate volume (hereinafter master/duplicate volume), contains content identical to the duplicate volume (known as the old data). In resynchronization of the master/duplicate volume, the main storage transfers compressed exclusive logical sum (XOR) data to the subsidiary storage in which XOR data generated by calculating the XOR for the old data in the copy volume and the data in the master volume (new data which contains data written in since the previous synchronization) has been compressed, the new data being replicated on the subsidiary storage side using the compressed XOR data that has been transferred.
Where few parts have changed between the old and new data, the XOR data contains a large number of “0” bit values. In other words, it can be said that the quantity of information (entropy in the information theory) in the XOR data is small compared to the new data. Thus the size of the compressed XOR data is less than the size of the compressed new data, and the quantity of data transferred via the WAN can be reduced.